1. Field of the Invention
Embodiments of the invention relate to a temperature controller. More particularly, embodiments of the invention relate to a device adapted to prevent a heater from overheating.
This application claims priority to Korean Patent Application No. 2005-0032659, filed on Apr. 20, 2005, the subject matter of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
In general, semiconductor devices are fabricated through a complicated series of fabrication processes. Various fabrication processes use various kinds of chemicals and chemical application techniques. One chemical application technique uses a bath filled with the chemical. An etching process is one type of semiconductor fabrication process commonly adapted for use with a chemical bath. Following an etching process the chemical bath is frequently used to remove unwanted films and generally clean the surface of a wafer being processed.
The application of cleaning and/or etching chemicals to a wafer is often accomplished using a so-called “overflow method” in which pure (e.g., uncontaminated) chemical flowing from a chemical bath pour over the wafer. Alternatively, a wafer to be cleaned may undergo a two-step cleaning process including immersion in a first bath (a so-called Quick Drop Rinse (QDR) bath) followed by a second (final) bath.
In a cleaning or wet-etching process performed using a chemical bath, the etching rate achieved by the process is generally proportional to the temperature of the chemical in the chemical bath, so etching rates change in accordance with changes in the temperature of the chemical. Therefore, changes in the chemical's temperature may result in over-etching or under-etching during the etching process which can have serious negative effects on the quality of the products being fabricated. Such results may, however, be avoided by uniformly maintaining the temperature of the chemical bath during the cleaning or wet-etching process.
One conventional chemical temperature control device senses the temperature of the chemical periodically and, in accordance with whether the sensed temperature of the chemical is within the desired temperature range (hereafter, the “control temperature”) for the process being performed, performs the appropriate operation(s) required to maintain the chemical at the control temperature.
In one common temperature control device, the temperature of the chemical is determined by sensing a resistance change in a temperature sensor. To maintain the temperature of the chemical at a uniform level, when the sensed temperature is lower than the control temperature, the temperature control device supplies power to turn ON a heater until the control temperature is reached. In contrast, so long as the sensed temperature remains higher than the control temperature, the temperature control device does not turn ON the heater.
Thus, in the conventional temperature control device, the heater is turned ON and OFF in accordance with the temperature sensed by the temperature sensor. In the conventional temperature control device, the ON-state of the heater is termed the “reverse operation,” while the OFF-state of the heater, during which state the chemical bath is allowed to normally cool, is termed the “normal operation.”
The electrical current (e.g., the power) supplied to the heater is characterized by the well known relationship; Current (I)=Voltage (V)/Resistance (R), wherein resistance (R) and current (I) are inversely proportional. In the reverse operation, as the resistance of the temperature sensor increases, current falls and the temperature bias increases. In the normal operation, as the resistance of the temperature sensor decreases, current increases and the temperature bias decreases. As used herein, the term “temperature bias” refers to the amount by which the temperature sensed by the temperature sensor is greater than the actual temperature of the chemical bath.
In one possibility, the specific resistance of the temperature sensor may increase due to corrosion or some environmental change, for example. In such cases, the temperature bias will increase accordingly. For example, when the specific resistance of the temperature sensor increases by 3.8 Ω, temperature bias increases by 10° C. When an increase in the specific resistance of the temperature sensor causes the temperature “sensed” by the temperature sensor to increase erroneously without a corresponding increase in the actual temperature of the chemical bath, the actual temperature of the chemical bath will fall below the temperature supposedly sensed by the temperature sensor. That is, the temperature sensor may report that the chemical bath has attained the desired control temperature when it has not.
In the other possibility, the specific resistance of the temperature sensor may decrease due to corrosion or some environmental change. In such cases, temperature bias will decrease accordingly. When a decrease in the specific resistance of the temperature sensor causes the “sensed” temperature to be less than the actual temperature of the chemical bath, the actual temperature of the chemical bath may rise well above the control temperature without being properly indicated by the temperature sensor. Thus, the temperature sensor may report that the chemical bath has not attained the control temperature when is actually has, and may have actually exceeded it.
In either case, when the temperature sensor erroneously indicates that the chemical has attained the control temperature when is, in fact, is actually higher or lower than the control temperature, fabrication errors (e.g., under-etching or over-etching) may occur in the semiconductor fabrication process, which may produce defects in the semiconductor device being processed.
One conventional temperature control device adapted for use within a wet etching or cleaning station is disclosed, for example, in U.S. Pat. No. 6,059,986. In this conventional temperature control device, an alarm is generated visually and aurally to prevent fabrication errors that might otherwise occur when a heater and a power supply control unit are short-circuited due to corrosion.
In this type of conventional chemical temperature control device, if a chemical circulating pipe or pump is damaged and, as a result, the chemical cannot circulate through a chemical circulating pipe, the temperature of the chemical bath will fall. In response, a heater may heat an associated heating pipe (i.e., a portion of the chemical circulating pipe). However, because of the damaged pump or pipe, the chemical is not circulating through the heating pipe. Because the chemical is not circulating through the heating pipe, the temperature of the chemical in the heating pipe continues to rise while the bulk of the chemical bath cools to room temperature. When this situation occurs, the temperature sensor, which is disposed in relation to the heater, senses the heat generated by the heater, and when the heater overheats its control point, an associated controller generates an interlock control signal to interrupt power being supplied to the heater.
Figure (FIG.) 1 illustrates a temperature control device and its relation to a wet etching or cleaning station having the functionality described above in accordance with a conventional temperature control device. The conventional temperature control device of FIG. 1 will now be described in some additional detail.
A chemical bath 10 contains chemical solution, and the chemical solution in chemical bath 10 is circulated through pipe 28 and back to chemical bath 10 in the direction indicated by the arrows disposed along the inner side of pipe 28 in FIG. 1. A pump 20 disposed around pipe 28 performs a pumping operation to circulate the chemical solution of chemical bath 10 through pipe 28. As pump 20 pumps the chemical solution, a heater 22, which is separated from pump 20, heats a section of pipe 28, and thereby heats the chemical solution circulating through pipe 28. In this manner, the chemical solution may attain the desired control temperature. The control temperature provided by heater 22 for one selected fabrication process ranges from between about 250° C. to 400° C.
In the foregoing configuration, a first temperature sensor 12 is disposed in chemical bath 10, senses the temperature of the chemical solution in chemical bath 10, and provides this information to chemical temperature controller 14. Chemical temperature controller 14 compares the sensed chemical solution temperature with a control (or predetermined reference) temperature, and outputs a chemical solution temperature control signal, which may be either a power supply control signal or a power cutoff control signal, to a power supply controller 18.
When the sensed temperature is lower than the control temperature, chemical temperature controller 14 provides the power supply control signal; and thus, power supply controller 18 begins or continues to supply power to heater 22. When the sensed temperature is higher than the control temperature, chemical temperature controller 14 provides the power cutoff control signal to power supply controller 18; and thus, power supply controller 18 stops supplying power to heater 22.
A power supply part 16 selectively outputs power received from an Alternating Current (AC) power source in response to an interlock control signal. Power supply part 16 comprises a power switch and provides power to power supply controller 18 when the power switch is turned ON. Power supply controller 18 may be realized as, for example, by use of a silicon controlled rectifier (SCR). As described previously, power supply controller 18 selectively supplies the power received from power supply part 16 to a heater 22 in response to the chemical solution temperature control signal. When power supply controller 18 supplies power to heater 22, the temperature of heater 22 increases. When power supply controller 18 does not supply power to heater 22, the temperature of heater 22 falls.
Through these operations, heater 22 heats or stops heating pipe 28 in accordance with the temperature sensed by first temperature sensor 12. In other words, if the temperature sensed by first temperature sensor 12 (i.e., the sensed chemical solution temperature) has not reached the control temperature (i.e., a temperature ascribed to the desired fabrication process which in several common embodiments ranges between about 70° C. to 150° C.), heater 22 heats the heating pipe (e.g., a portion of pipe 28 disposed within heater 22) in accordance with the operation of power supply controller 18.
However, if the temperature sensed by first temperature sensor 12 has reached the control temperature, then heater 22 temporarily stops heating the heating pipe in accordance with the operation of power supply controller 18. The temperature of the chemical solution in chemical bath 10 is controlled as necessary through repeated application of the operations described above.
A second temperature sensor 24 is disposed on an external face of the heating pipe disposed within heater 22. Second temperature sensor 24 senses the temperature of heater 22 and provides a corresponding sensed-heater temperature to a heater temperature controller 26. Heater temperature controller 26 compares the sensed-heater temperature with a predetermined heater reference temperature (i.e., a heater overheating prevention reference temperature), and, when the sensed-heater temperature exceeds the predetermined heater reference temperature, provides an interlock control signal to power supply part 16 to stop power supply part 16 from supplying power to power supply controller 18.
However, in the conventional chemical temperature control device, if chemical solution is not circulating through pipe 28 because pipe 28 is damaged or clogged, or because pump 20 has broken down, the temperature of the chemical solution in chemical bath 10 will fall while heater 22 continues to heat the heating pipe. Under such circumstances the temperature of heater 22 and the heated pipe may increase very dramatically. Then, if second temperature sensor 24 disposed in heater 22 corrodes or a sensing line is short circuited, heater temperature controller 26 will not generate the interlock control signal, and power supply part 16 will continue to supply power to power supply controller 18. Thus, power supply controller 18 will continue to supply power to heater 22, and heater 22 will overheat. In extreme cases, overheating may actually cause a fire.